1. Technical Field of the Invention
The present invention relates to cellular communications systems and, in particular, to the transmission of a message instructing a switching node to release a previously allocated and reserved communications resource.
2. Description of Related Art
When an incoming call (voice, data or fax) is dialed to a mobile station, the cellular communications system responds by attempting to locate the mobile station using a paging process. In particular, a visited switching node (also referred to as the visited mobile switching center (V-MSC)) believed to be currently serving the called mobile station has one or more of its base stations broadcast paging messages over the air interface. These paging messages are addressed for the called mobile station. If this paging is unsuccessful, other neighboring switching nodes may also make paging broadcasts. If the mobile station responds to any one of the broadcast pages, it has been located, and a communications resource (comprising a traffic channel) over the air interface is reserved to handle delivery of the incoming call. The mobile station is further marked by the communications system as being busy. The cellular communications system then routes (through-connects) the incoming call to the serving switching node for delivery over the reserved communications resource to the called mobile station. Once the incoming call is routed to the serving switching node, a call alert notification (typically comprising a ring) is provided to the subscriber.
While the call is being delivered by the communications system implementing such a page before routing procedure, the mobile station remains connected to the reserved resource. This call routing process may take a time period of several seconds to complete. During this time period, the communications resource remains engaged (i.e., reserved) solely for the use of the called mobile station, and is thus not available for use by other mobile stations. Furthermore, during the routing time, the subscriber owning the mobile station cannot either make or receive another call. In this regard, it should be remembered that although the communications system is aware of the incoming call to the mobile station, the subscriber owning that mobile station has not yet been notified (through, for example, a call alert or other message) that an incoming call is being through-connected for delivery over the reserved resource.
In many instances, delivery of the incoming call to the mobile station fails even though the paging process successfully located the mobile station. For example, the calling party may drop the call before the delivery to serving switching node is completed. In commonly assigned, co-pending application for patent Ser. No. 08/510,972, entitled "System and Method for Addressing a Release Resource Message" filed Aug. 3, 1995 (the disclosure of which is hereby incorporated by reference), a release resource message is sent to the visited switching node when the calling party drops the call or access is denied to the called mobile station before call delivery is completed. This allows the reserved resource to be made available to other mobile stations in connection with performing a call delivery.